A size of a package substrate increases with further high functionalization of an information apparatus and the increased number of pins of the package substrate. A semiconductor element is miniaturized, and the semiconductor element has low voltage and high current. A semiconductor package includes the package substrate and a semiconductor chip mounted on the upper surface of the package substrate. There is a ball grid array (BGA) as one of semiconductor package structures. In the semiconductor package of a BGA type, BGA balls (solder balls) are disposed in a matrix manner on the lower surface of the package substrate. In a case where the semiconductor package is installed on a printed circuit board (PCB), the semiconductor package is electrically connected to the printed circuit board through the BGA balls.
As an example of specifications of the package substrate, there are a substrate size: 50 mm×50 mm, a BGA pitch (BGA ball pitch): 1.0 mm, and a BGA ball size: ϕ 0.6 mm. As an example of specifications of a large package substrate, there are the substrate size: 76 mm×76 mm, the BGA pitch: 0.8 mm, and the BGA ball size: ϕ 0.5 mm. In the large semiconductor package, the substrate size is increased by changing the substrate size of the package substrate from 50 mm×50 mm to 76 mm×76 mm, and the BGA pitch is miniaturized by changing the BGA pitch from 1.0 mm to 0.8 mm. The semiconductor element installed on the package substrate is multi-cored, and a large current is supplied to the semiconductor element.
As a power supplying method from the package substrate to the semiconductor element, there is a horizontal power supplying method that supplies the power to the semiconductor element through wiring mounted in the package substrate and extending in the planar direction of the package substrate. In the case of the horizontal power supplying method, the number of core layers of the package substrate increases and the number of layers of the printed circuit board increases according to the supply of the large current. Therefore, when the power is supplied from the package substrate to the semiconductor element, a power supply drop increases. In order to reduce the power supply drop, a vertical power supplying method that supplies the power to the semiconductor element from a direction directly below the semiconductor element is employed.
As illustrated in FIG. 17, in a structure of a semiconductor package 90 of the vertical power supplying method, Cu pillars 81 for current supply are mounted under a printed substrate 80 and at a position directly below a semiconductor element 91 and electrical connection from the Cu pillars 81 to BGA balls 82 and 83 through the printed substrate 80 is performed to supply the power to the semiconductor element 91. The power supply drop when the power is supplied from a package substrate 92 to the semiconductor element 91 is reduced by employing the structure of the semiconductor package 90 of the vertical power supplying method.
In the semiconductor package 90 of the vertical power supplying method, for example, the semiconductor element 91 is installed on the center portion of the upper surface of the package substrate 92, and IO parts 93 are installed on the outer peripheral portion of the upper surface of the package substrate 92. The power to the semiconductor element 91 is supplied from the package substrate 92 through the BGA balls 82, and the power to the IO parts 93 is supplied from the package substrate 92 through the BGA balls 83. In order to satisfy an allowable current of the BGA ball 82 used for supplying the power to the semiconductor element 91, a size of the BGA ball 82 used for supplying the power to the semiconductor element 91 is larger than a size of the BGA ball 83 used for supplying the power to the IO parts 93. Therefore, a size of a land bonded to the BGA ball 82 for supplying the power to the semiconductor element 91 is designed to be larger than a size of a land bonded to the BGA ball 83 used for supplying the power to the IO parts 93. In FIG. 17, an illustration of the land is omitted, but the land is mounted on the lower surface of the package substrate 92. A part of the land is covered by resist, and the size of the land is a size of a portion of the land exposed from the resist.
FIGS. 18 to 23 are cross-sectional views of a package substrate 101 according to a comparative example. As illustrated in FIG. 18, BGA balls 103 are mounted on a land 102 mounted on the package substrate 101. In FIGS. 18 to 23, the lower surface of the package substrate 101 faces the upper side. A part of the land 102 is covered by a resist 104 mounted on the package substrate 101. A flux 105 is coated on the land 102. When the BGA balls 103 are mounted on the land 102, a position deviation of the BGA ball 103 may occur. In addition, as illustrated in FIG. 19, there is a case where the BGA balls 103 move and the adjacent BGA balls 103 approach each other as the flux 105 spreads after heat treatment is started and before the BGA balls 103 are melted.
When a size of the land 102 increases, a movement amount of the BGA ball 103 increases. As illustrated in FIG. 20, there is a case where the BGA balls 103 are connected to each other by the melting of the BGA balls 103 in a state where the adjacent BGA balls 103 approach each other during the heat treatment. In FIG. 21, a shape of the BGA ball 103 after the end of the heat treatment is indicated in the state where the adjacent BGA balls 103 are connected to each other. As illustrated in FIG. 21, the BGA ball 103 spreads on the land 102 in the state where the adjacent BGA balls 103 are connected to each other.
As illustrated in FIG. 22, there is a case where one of the adjacent BGA balls 103 is drawn into the other of the adjacent BGA balls 103 by the melting of the BGA balls 103 in the state where the adjacent BGA balls 103 approach each other during the heat treatment. In FIG. 23, a shape of the BGA ball 103 after the end of the heat treatment is indicated in the state where one of the adjacent BGA balls 103 is drawn into the other of the adjacent BGA balls 103. As illustrated in FIG. 23, a height of the BGA ball 103 increases and a place where the BGA ball 103 does not reach the land 102 is generated.
In a case where the height of the BGA ball 103 is lower than a specified value or the place where the BGA ball 103 does not reach the land 102 is generated by connecting the adjacent BGA balls 103 to each other, correction work such as removing the BGA ball 103 from the land 102 occurs and a lot of man-hours are desirable for the correction work. In addition, there is a case where the land 102 is peeled by the correction work leading to a failure of the package substrate 101. Therefore, it can be considered that a size of the BGA ball 103 is decreased to suppress the connection between the BGA balls 103. However, when the size of the BGA ball 103 is decreased, an allowable current value of the BGA ball 103 is reduced.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 11-74637,
[Document 2] Japanese Laid-open Patent Publication No. 2010-161205, and
[Document 3] Japanese Laid-open Patent Publication No. 2011-119765.